This invention relates to amplitude modulation systems and more particularly to vestigial sideband amplitude modulation systems such as are used in television broadcasting transmitters.
It has been found in efforts to use present state-of-the-art solid state linear amplifiers in the low-level drive stages of a television transmitter that these solid state amplifiers produce large amounts of incidental, unwanted phase modulation. It has been suggested that, by the use of a phase-to-amplitude modulation system, amplifiers with a greatly reduced linearity requirement can be used in the low-level drive stages.
A phase-to-amplitude modulation system is described in a paper published in the Electronics Magazine of September 1950 on pages 102 through 106. The paper is entitled "Phase to Amplitude Modulation" and is by William E. Evans, Jr. Although Evans suggests this type of system for television broadcasting, applicant knows of no use of this form of modulation in television broadcasting other than the referenced experimental use. Evans does not discuss the use of vestigial sideband filtering, and, since such filtering is conventionally done at high power levels just before coupling to the antenna, it is assumed that Evans contemplated the provision of sideband filtering at high power levels after the combiner. As will be discussed, it is desirable, particularly from the standpoint of filter cost and/or performance, that vestigial sideband filtering be done at a low power level. Further it is desirable that all signal processing be done at a fixed frequency with translation to final television channel frequencies. This approach permits the use of a common signal processor for all television channels, lowering total cost and increasing reliability.